Hi, I'm new but I guess I might as well share my thoughts on it as well, since there are obviously reasons why I registered here today.
1. I don't think you need to make games itch.io exclusive, BUT what motivated me to give itch.io a look was when a dev told me that he gets the most money if I buy the game from itch.io. I feel devs should always point it out (e.g. on their websites) that this is their preferred platform. Or even point it out at all, because I've seen several websites that ONLY linked to Steam even if the game was also available on other stores.
2. For me Steam is usually not an option because I only play DRM-free. A dev mentioning "you can also get the game DRM-free here: ..." would definitely also be a reason to check out that platform for me and other DRM haters. Many other stores only sell Steam keys after all!
3. Generally, games on itch.io are REALLY hard to find. This also needs work from the itch.io staff, but what devs can always do is spreading their games more. If you never read about them, you don't know you want to play them! It doesn't even need to be paid ads. Just talking about your games in various forums already works (e.g. if I see a new game mentioned on one of the subreddits I frequently check, I usually tend to always check it out).
4. Make it easier to find your games for people who actually already visit itch.io to give it a try! Make proper use of tags for example that allow a more granular search. Right now when I visit itch.io, I feel like I'm seeing only games I'm not interested in and even trying to filter/search somehow still yields a lot of titles that don't really fit into what I'm looking for (while other titles I'm actually interested in are no longer visible because they aren't marked with any tags at all). Not finding the right games for you, really demotivates you from using this website.
(5.) Other than that I think there's needs to be done more by the staff:
a) A more visible rating system (there seems to be some sort of rating, but it seems like an arbitrary number as it doesn't seem to link to anything - how do you even rate and why are some of the most popular games outside itch.io so far down the list on here? also where are the reviews? also why do I only see a rating when I sort by it? I want to always see the rating)
b) Quick access to the "good stuff" (needs to be visible directly to new visitors, because the first games people see need to look good and the first games people try need to be good or they assume games here are not of the quality they seek; I'm not talking about AAA titles but titles that after you played them would be considered a hidden gem by a large audience - right now no matter how I sort, I feel that the top entries are very niche games that are super popular among only a very specific audience)
c) The ability to search for keywords and then still be able to sort/filter would really help...
d) Maybe give some insight on what the sorting even means - what makes a game popular? does top seller go by money earned, number of products bought, number of products downloaded (I find it rather strange because if it would be downloads, I'd assume all 50% of the free games coming first before any of the paid ones, but it seems to be quite mixed)?
e) I may be wrong on that but I feel that "in dev" games are completely mixed with finished products on here and there's no filter for only finished games. It would be cool to have some more hand-picked list akin to "these are latest fully finished games that we consider of extraordinary quality", similar to the feature games list that's already there (I guess the best way to go right now would be to set the tags you like then filter for featured games only and then sort by best rated - but that includes a lot of setting up and the featured filter is actually really well hidden)
f) Inverted tag search would also be nice ("all games except games that have tag X").
g) Allow games to marked as "Like" even if you haven't purchased them on itch.io, so it is considered for you personal recommendations list.
Well, I guess those are my thoughts on it.
Edit: I guess I have another suggestion but can't judge itch.io on that yet:
6) Stay in contact with your players! I know this from GoG - a lot where devs would release their games on there, but then not talk with their players at all and completely ignore their game's forums (and instead only reply and post on Steam). This is very discouraging and makes you want to use the website less.